Described below is a voltage-limiting composition, having a polymer matrix and a particulate filler containing partially conductive material. Also described is a body with such a voltage-limiting composition and a method for producing a voltage-limiting composition. The voltage-limiting composition can be used particularly advantageously for surge arresters with a varistor property, in particular in medium-voltage systems, low-voltage systems, cable connections and cable fasteners.
Non-linear resistors with varistor behavior are used in a wide variety of applications, such as in voltage protection devices, in cable connections, in cable terminations, etc. These non-linear resistors may contain a polymer matrix and a filler embedded in the polymer matrix. The filler contains sintered pellets with predominantly spherical particles of doped metal oxide (metal-oxide varistor (MOV) filler). The particles of the pellets have crystalline composite granular structures. Since a varistor with a polymer matrix/MOV filler composite material can be produced much more easily than a comparably acting varistor on the basis of a sintered ceramic, varistors can be produced with the composite material comparatively easily, inexpensively and in a great variety of forms.
The viscosity of the composite material increases with increasing filler content, until, depending on the matrix material and the type of processing, a practical upper limit is reached, for example of about 50 percent by weight in the case of casting resins. In the case of lacquers and in the case of the production of prepregs, the viscosity of the composite material is lowered by adding solvent until ideal processability is achieved.
On account of the high density of doped metal oxides, of typically more than 5 kg/m3, and the relatively low density of the polymer of the polymer matrix, of about 0.8 to 1.3 kg/m3, the filler quickly settles, as a result of which the maximum degree of filling in the system is restricted, or makes a certain type of processing virtually impossible. For instance, lacquers and prepregs with a low viscosity and partially conductive fillers have not so far been usable in practice.
WO 97/26693 describes such a polymer matrix/MOV filler composite material. Powdered metal oxide pellets, which are produced by sintering a spray-dried MOV powder on the basis of a zinc oxide doped with oxides of Bi, Sb, Mn, Co, Al and/or further metals is used as the filler. These pellets have spherical particles formed like a soccer ball, with varistor behavior. The particles have a diameter of up to 125 micrometers and vary in their size with a Gaussian distribution. This material is used in cable connections and cable terminations, where it forms voltage-controlling layers.